Plant community change mediated heterotrophic respiration increase explains soil organic carbon loss before moderate degradation of alpine meadow

Abstract Alpine meadows on the Qinghai‐Tibet Plateau (QTP) store a huge amount of plant and soil carbon (C). The degradation of the alpine meadow has led to a significant loss of soil organic C (SOC), which endangers its weak C sink. The change in heterotrophic respiration was hypothesized as one of the primary biological processes for the SOC loss alongside the degradation of alpine meadow. However, little is known about how land degradation impacts R h due to changes in plant communities and consequent labile organic C inputs. R h , vegetation productivity, plant community composition, plant lignin concentration, leaf C/N, SOC, and soil labile C (SLC) were measured for alpine meadow grasslands with five degradation levels in 2 years. Our results showed that the SOC of the surface layer (0–10 cm) gradually decreased with land degradation. The plant lignin concentration and C/N ratio gradually increased with land degradation due to the proportion of sedges decreased while that of forbs increased, which consequently led to the soil C quality (SLC) increased before the moderate degradation level (MD) while significantly reduced in severe and very severe degradation levels (SD and VSD). The structural equation modeling result (SEM) revealed that the degradation‐induced change in soil C quality (SLC) due to an alteration in plant community composition relates to the R h change with grassland degradation, and the R h (C output) and aboveground biomass (AGB: C input) further mediated the loss of SOC alongside alpine meadow degradation. On average, R h increased by 49% and 59% from the Intact to slight degradation level (SLD) and MD and reduced by 56% and 69% from MD to SD and VSD. The temperature sensitivity of R h ( Q 10 ) was higher in the SLD ( Q 10 = 2.84) and MD ( Q 10 = 2.62) comparing with the Intact ( Q 10 = 2.58) respectively, but it was lower in SD ( Q 10 = 2.19) and VSD ( Q 10 = 1.65). AGB and belowground net primary productivity (BNPP) had no significant change until the MD, while reduced by 60%–90% in SD and VSD. Our results suggest that R h and its Q 10 change moderated by soil C quality alteration due to the plant community shifting could be one mechanism for the soil C loss before MD, but could not be the main pathway for the SOC loss at SD and VSD.